1. Technical Field
This invention relates to a light beam deflecting device provided for an image recording device such as a laser printer and a digital copier and which employs a rotary polygon mirror to deflect the output laser beam of a laser beam generator, and more particularly to an improvement of such a light beam deflecting device.
2. Background Art
A conventional light beam deflecting device of this type is designed, for instance, as follows: In the device, a laser beam outputted by a semiconductor laser is applied through a condenser lens to a rotary polygon mirror, so that it is deflected while being reflected by the rotary polygon mirror, so as to scan a photoreceptor drum axially with the aid of an image forming lens.
The rotary polygon mirror is connected directly to a drive motor; that is, it is driven directly by the motor. The rotary shaft of the drive motor is rotatably supported by ball bearings.
It is well known in the art that the ball bearings supporting the rotary shaft of the rotary polygon mirror vibrate as the rotary shaft rotates (cf. "Vibration Engineering Handbook", pp. 973 to 986).
The vibrations of the ball bearings attribute, for instance, to the springs of them, to the manufacture of them, and to unsatisfactory handling of them.
In the case where the ball bearings vibrate because of the springs of them, the vibrations are due to the passage of the rolling elements, to the elastic characteristic of the bearings, or to the critical speed of the rotary shaft. In the case where the vibrations of the ball bearings depend on the manufacture of the bearings, they are produced by the finished surface which is swelled, or they attribute to the vibrations of the holders of the ball bearings. Furthermore, in the case where the vibrations attribute to the unsatisfactory handling of the bearings, the latter have scratches, or dust is held therein.
When the ball bearings supporting the rotary shaft of the rotary polygon mirror are vibrated in the above-described manner, they produce sounds with frequencies in the audio frequency band audible to the ear. The acoustic vibration in the audio frequency band causes the light beam deflecting device to make noise to some extent. The acoustic vibration occurs steadily when the rotary shaft is turned at high speed, and therefore it is not so serious when it is turned at a low speed of the order of 2000 rpm, or when the device is used alone.
The ball bearings supporting the rotary shaft of the rotary polygon mirror are turned at high speed, and a plurality of the same ball bearings are employed for supporting the rotary shaft. Hence, the acoustic vibrations in the audio frequency band of those ball bearings are equal in frequency and different in phase as shown in FIGS. 9(a) to 9(c). Therefore, the acoustic vibrations interfere with one another, thus producing beat sounds. The beat sounds thus produced are offensive to the ear, varying in amplitude with time. When the beat sounds are produced, particularly in an image recording apparatus such as a laser printer, the recorded image is adversely affected thereby because the beat sounds vibrate the laser beam which is deflected by the rotary polygon mirror in a scanning mode.
In general, an image recording apparatus such as a laser printer is set on a desk in an office or the like when used. Therefore, the beat sounds produced by the image recording apparatus will be offensive to the ear even if they are small in sound volume.
In order to eliminate the difficulty that the ball bearings produce beat sounds in the above-described manner, techniques have been proposed by Japanese Patent Unexamined Publication (OPI) Nos. Sho. 61-151881 and Sho. 64-74316 (the term "OPI" as used herein means an "unexamined published application").
The technique proposed by Japanese Patent Unexamined Publication (OPI) No. Sho. 61-151881 concerns a magnetic disk device comprising a magnetic disk and a magnetic head. In the magnetic disk device, a needle bearing and a ball bearing are provided for a rotary actuator adapted to position the magnetic head with respect to the magnetic disk, and therefore the acoustic vibrations produced by those bearings are different in frequency from one another, which prevents the production of beat sounds.
The technique proposed by Japanese Patent Unexamined Publication (OPI) No. Sho. 64-74316 concerns a disk type memory device including a spindle assembly which comprises: a spindle; and at least one rolling bearing which rotatably supports the spindle. In the device, the number of rolling elements in the rolling bearing is set substantially to an even-number times the ratio of the diameter of the outer or inner raceway track of the rolling bearing to the diameter of the pitch circle of the rolling elements, so that the rotation-asynchronous vibration is decreased.
In the above-described magnetic disk device, production of the beat sounds can be prevented to some extent. However, it is not practical to apply the device to a light beam deflecting device in which a rotary polygon mirror is rotated at high speed, about 20,000 rpm, because the device employs the needle bearing.
The disk type memory device is advantageous in that the rotation-asynchronous vibration is decreased; however, it still suffers from a difficulty that production of the beat sounds cannot be effectively prevented.